1. Field of the Invention
The present invention relates to a method for forming a semiconductor. Particularly, the present invention relates to a method for forming a semiconductor film, which has an excellent crystalline property and contains less impurity in a crystalline film. The semiconductor film prepared in accordance with the method of the present invention can be used in such semiconductor device as a high quality thin film transistor having a high field effect mobility.
2. Description of the Related Art
So far, a method for crystallization of a non-crystalline silicon hydride film formed by a plasma CVD or a heat CVD method, which contains a lot of hydrogen, and a lot of bonding hands of silicon which are neutralized with hydrogen (it can be described as a-Si:H), by irradiating such laser light as CW (Continuous-wave) laser and Excimer laser has been well known.
This method, however, poses a problem that a non-crystalline silicon film as a starting film contains a lot of hydrogen, much of which spouts from the film when the film is subjected to a laser irradiation, and then a film quality deteriorates remarkably. To overcome this problem, there have been used chiefly the following three methods.
(A) At first, by irradiating a low energy density (less than threshold value energy for crystallization) laser light to a non-crystalline silicon hydride film as a sample, hydrogen in the starting film as a sample will be drawn out. And then, by irradiating a high energy density laser light to the sample, the sample will be crystallized. This is called as a multi-stage irradiation method.
(B) By forming a non-crystalline silicon hydride film, at 400° C. or more of a substrate temperature, a hydrogen content in the starting film will be decreased. Thus, a film quality deterioration by a laser irradiation will be prevented.
(C) By effecting a heat treatment of a non-crystalline silicon hydride film, in an inactive atmosphere, hydrogen in the film will be removed.
The following problems, however, have been posed, in order to obtain a high quality crystal film (in general, polycrystal silicon film) by the above mentioned methods.
(1) The method of (A) has a problem in a practical use. Namely, it is bad in an efficiency of hydrogen drawing out, difficult to control a laser energy output, and required to increase the number of laser irradiation times. Since almost all the laser light energy is absorbed in the film surface, it is difficult to draw out hydrogen, in case of a thick film.
(2) The method of (B) has a problem that a crystallization is obstructed in a later process of crystallization by a laser light. This is caused by that an impurity content in the film is inclined to increase, when a substrate temperature becomes high, and a silicon cluster (microcrystal part of silicon) is formed, if a non-crystalline silicon hydride film is formed at a high temperature.
(3) The method of (c) has a problem that an electric property (carrier mobility etc.) of the crystal film deteriorates, caused by such impurity as oxygen. This is derived from the reason that since hydrogen is eliminated by heating, a dangling bond (unpaired bonding hand) of silicon is formed, and the dangling bond combines easily with oxygen etc. then, the oxygen intrudes easily into the film from its surface to 10 to 20 nm in depth, and diffuses into the film deeply by a high temperature diffusion.